The invention relates to a process and a production installation for producing shell-shaped, fiber-reinforced plastic parts, in particular from two-component synthetic resin.
In a contribution at the plastics engineering conference “PUR-Technik 1996” in Munich from 31.01.-1.02.1996, Kleinholz et al. discussed a process under the title “Trägerteile für Automobilinnenverkleidungen aus naturverstärktem PUR-Harz” (supporting parts for automobile interior trim of natural-fiber-reinforced polyurethane resin); the corresponding discourse is printed in the conference report issued at the time (PUR-Technik 1996, VDI-Verlag, ISDN 3-18-234193-6) on pages 17–30. The known process is intended to produce unfinished parts for interior door trim of passenger car doors in which natural fiber mats (flax, sisal, jute and so on) are embedded in polyurethane (PU) as the synthetic matrix resin. The fiber mats are formed by a number of nonwoven fabrics formed from broken down natural fibers, in which the individual natural fibers are contained in a random laid layer, are matted by needles to form a uniform fiber composite and are supplied by the mat manufacturer in storage rolls.
During the production of interior door trim, blank parts made to match the dimensions of the trim concerned are cut off from the fiber mat web and initially conditioned to a specific dry content of the natural fibers. The components for the polyurethane resin, mixed in a defined way, are sprayed onto the flat blank in a specifically set amount per unit area by a robot, it being possible for the resin coating to vary locally according to local component stressing. The resin-coated mat blank is manually placed into the female mold of the open forming press, insert parts consisting of rigid plastic on which fastening points of the interior door trim are formed having previously been placed into the female mold, likewise manually.
During pressing, they are bonded with the unfinished part, the polyurethane resin serving as an adhesive. The pressing operation takes place at temperatures above 100° C. and lasts approximately 40 to 70 seconds. After the pressing, the unfinished part can be removed, but still has to be cooled down to room temperature in a separate cooling device in order to avoid distortion. However, only nonwoven fabrics with high elasticity can be formed without any folds to any appreciable extent by this process. The workpiece produced in the cited literature reference—interior door trim for passenger car doors—does not demand high requirements in this respect. Non-elastic fiber mats of ordered long fibers extending continuously and rectilinearly over the entire fiber mat can at most be formed into workpieces with little profiling by this process. If workpieces with greater profiling are to be produced by this process using non-elastic fiber mats, these fiber mats must be previously processed into preforms adapted to the workpiece in a separate process. However, this presupposes that the fibers of the mats are bonded with polymer at the interstices of the mutual crossing points. However, such encapsulation of the fibers in thermoplastic material makes the fiber mats more expensive.
In the book published by the VDI-Verlag for the Institut für Kunststoffverarbeitung (Institute for plastics processing), Aachen, “RTM/SRIM: Serien-fertigung von Faserverbundbauteilen” (RTM/SRIM: Series production of fiber composite components) (ISBN 3-18-990015-9), contribution 2 by A. Bruns “Entwicklung und Produktion von Bauteilen im Harzinjektionsverfahren RTM” (development and production of components by the RTM resin injection process) (pages 28–36) describes inter alia under the heading Anwendungsbeispiele (application examples) an installation for producing outer shells of protective cycle helmets. Reinforcing blanks of a fiber mat of polyethylene fibers are introduced in a dry state into an open female mold of a forming tool. If required, insert parts which reproduce locally finer structures of the later workpiece can be placed in advance into the female mold. By closing the forming tool, the fiber mat blank is draped into the spatial form defined by the form of the tool. In order that the blank can be placed into the form without any folds, it must be provided at suitable points with incisions, or a number of blanks overlapping at the edges, each only partially covering the shell structure, must be used.
After closing the forming tool, a reactive resin mixture of a thermosetting-curable synthetic resin (for example epoxy resin (EP), vinyl ester (VE), unsaturated polyester (UP) and so on) is injected into the fiber structure of the mat, for example by the vacuum injection process, and the placed-in blank parts of the fiber mat are consequently impregnated. The matrix resin can then cure.
To increase annual production, the forming press has three tool sets, so that three helmet shells can be produced virtually simultaneously in one cycle. Moreover, each tool set comprises two lower parts (female mold) and one upper part (male mold), the two female molds of a tool set being able to move from opposite sides of the forming press into the latter, under the male mold. On account of such a configuration of the forming press, during the reaction time of one workpiece, the other female mold can be prepared for a new operating cycle, i.e. cleaned and loaded with new parts. The two female molds of a respective tool set can be alternately brought into the pressing position underneath the male mold, on the one hand, and into a laterally offset preparatory position, lying outside the press, on the other hand. This allows the reaction time of the resin to be used for preparing a new operating cycle. The disadvantage of the known RTM process is that the shell-shaped parts produced with it have an interrupted fiber reinforcement. If it is wished to avoid this, it is necessary—as already explained further above—for separately prefabricated preforms of more expensive fiber mats to be used.
In the same book, mentioned above, contribution 1 (pages 1–27) by Michaeli et al. “Preformherstellung-Umformen” (preform production-forming) is concerned inter alia with the technical aspects of an installation for producing preforms from highly porous mats of synthetic fibers. The mats—in particular loosely beaten woven fabrics or laid scrims of glass fibers or glass fiber rovings—are not only easily bendable but also intrinsically shearable with low resistance. Such a mat adapts itself—in a way similar to a metal sheet in a deep-drawing process—to an amazing extent to spatially defined surface contours if the mat is held under tensile stress from the edge during the forming. To be able to fully utilize the deforming potential of the fiber structures, two-part forming tools comprising a female mold and a male mold are necessary for forming. To be able to stabilize the fiber mats after their forming in the new form, the fibers of such forming mats are doped with a fine powder of a low-melting thermoplastic or a thermally reactive thermosetting material, this material preferably becoming lodged interstitially on the fiber circumference in the region of the crossing points of the fibers.
The binder is thermally activated by heating the mat during the forming and is stabilized by subsequent cooling. The preform produced in this way is stabilized only very weakly in its new shape and must be handled carefully. Its fiber mat is still highly porous and absorbent with respect to a matrix resin. In an example of an installation for the process described in this literature reference, an unwinding station with three storage rolls of fiber mat webs is provided, the individual webs of which run together to form a three-ply web. A shearing cutter beam which extends transversely over the web and can be driven back and forth is used to cut individual blanks to length from the said web in a way corresponding to a desired workpiece. In each case, a blank is heated in a heating station by radiation and/or convection to the softening temperature of the binding polymer, in the heated state is quickly clamped in a clamping frame and both—the clamping frame with the blank—are pushed into the forming press with the female mold and male mold for forming.
The clamping frame for fixing the edge of the mat is adapted to the component geometry and is designed with respect to its edge clamping in such a way that the clamped-in edge of the mat can slide out of the clamping restraint under tension. This is achieved by special friction elements which can be pressed with a defined clamping force, which can be predetermined by pneumatic cylinders, and make it possible for the fiber material to slide after itself during forming. The clamping force must be set such that the fibers do not tear (clamping force too great) and nor do the fiber mats become folded (clamping force too small) during forming. The fiber mat formed into a preform in this way must be kept in the closed tool until the fiber mat has cooled down again and the binding polymer has hardened at the points of intersection. The preform, which now has a certain rigidity of its own, and can be carefully handled manually or, for example, by needle grippers while retaining its form, can subsequently be removed from the lower tool and the clamping frame. Before further processing of the preform, for example in the process described above to form a fiber composite plastic part, the edge of the preform remaining in the clamping frame still has to be trimmed.
In the dissertation by U. P. Breuer: Beitrag zur Umformtechnik gewebeverstärkter Thermoplaste (contribution to the forming technique of fabric-reinforced thermoplastics), VDI Fortschrittberichte (VDI progress reports), series 2: Fertigungstechnik (production engineering) No. 433, VDI Verlag 1997 (ISBN 3-18-343302-8), there is described inter alia a process for producing shell-shaped, fiber-reinforced plastic parts from thermoplastic panels known as ‘Organobleche’. This semifinished product comprises sheet-like, virtually endless webs which consist of mats of inorganic fibers—in particular glass or carbon fibers—with a thermoplastic as the matrix resin, and are usually supplied in a coil to the further processor. From this, panels corresponding to the workpiece are cut off to the desired dimensions, heated to the flowing temperature of the thermoplastic matrix polymer and placed into a forming press. To be able to form the heated and softened panels between the female mold and male mold without any folds, it is kept in the stretched state during the forming, the respectively local stress state having to be empirically optimized workpiece-dependently.
Arranged around the female mold is a clamping frame, into which the edge of the softened panel is clamped. The clamping jaws take the form of a multiplicity of narrow rollers, each of which is respectively provided with an adjustable braking device. This makes it possible for the panel to slide after itself out of the edge clamping restraint under an adjustable tensile force during the forming. A disadvantage of this type of edge clamping is that the structural expenditure for this edge clamping is very great and accordingly not only is this quite expensive to produce but frequent disruptions are also likely during operation. What is more, it is not possible for the edge to be firmly held by the braking rollers without a gap. Rather, tension-free strips of varying width between neighboring braking rollers, where folds can form, are constructionally unavoidable.
The process and device shown in DE 198 29 352 A1 for producing shell-shaped moldings from fabric-reinforced plastic are also based on the so-called Organoblech as a preliminary or semifinished product. In the case of the process shown there, the forming operation producing the desired final form in the forming tool is preceded by a preforming step. This is consequently a two-stage forming process with preforming and final shaping. During the preforming, a punch which is brought only very approximately up to the engraving of the forming tool is pressed into the softened plastic panel stretched over the forming tool and the material is consequently made to approximate roughly to the engraving determining the form. Only subsequently is the material completely pressed against the engraving by a vacuum from below and positive pressure from above and the final form produced in this way in the shell material.
The compliant edge clamping of the fabric-reinforced plastic panel provided in the case of this process is also of a two-stage form. To be precise, during the preforming the plastic panel can slide after itself largely without friction out of the edge clamping restraint, whereas during the final shaping a constant tensile stress, counteracting deformation, can be exerted at the edges, allowing deforming largely without any folds. For this purpose, the edge of the panel is clamped between two frames, one of which is fixed in place in relation to the forming tool and the other of which is movable back and forth in relation to the forming tool in the closed state of the press. It is pressed against the lower frame with a force which is predetermined by springs and is small during the preforming and great during the final shaping. With such a pair of frames which can be pressed by spring force, however, only a tensile force that remains the same over the entire periphery of the edge of the panel can be exerted on the plastic panel during the forming. Moreover, this solution cannot be readily transferred to loose fiber mats which, on the one hand, behave anisotropically—unyielding in the direction of the filaments; yielding diagonally to the direction of the filaments—and, on the other hand, are subject to the risk of easily splaying open at the edge.
Problematical in the case of all processes based on loose fiber mats is the handling of the fiber mats. Although the relevant installations described are designed in such a way that the mat blanks, originating from at least one wound-up material web, are conveyed along a linear sequence of working stations following directly one after the other, it remains open how this actually takes place. It is conceivable for the blanks to be carried on flat conveyor belts or on a group comprising a number of conveying straps running parallel to one another. If, on the other hand, it is intended for example after heating for the mat blank to be quickly transferred into a clamping frame, this presents considerable constructional and operational problems. Moreover, closely strung-together production lines are, on the one hand, susceptible to faults on account of the rigid sequence of stations, because a fault in any one station shuts down the entire line. Therefore, the availability of such production lines is generally not optimal. On the other hand, a sequence of working stations that is rigidly connected in technical conveying terms is not very flexible and cannot be readily converted to a different product or a modified process sequence.
In the case of manual handling of the blanks, a production installation can indeed be designed and operated flexibly, but the mats or workpieces then have to be handled by people, which is not only monotonous and physically demanding, but is also expensive on account of the high number of personnel used, in particular in multishift operation.
A further problematical point in the case of the known processes or installations is the state at the edge of the blank parts, which is distorted or frayed to varying extents, it being possible for the fibers to be splayed to varying extents, in particular if—as usual—loosely bonded woven fabrics or laid scrims of superficially smooth fibers, for example glass fibers, are concerned. The edges of the blanks are often already splayed directly after cutting if the cutting means has become blunt. Fabric splaying at the edge is also often unavoidable when large blanks are handled manually. However, distorted or frayed edges of the blank parts can lead on the one hand to operational disruptions and on the other hand even to reject parts.
On the basis of the prior art described, it is the object of the invention to present a process and a production installation for producing shell-shaped, fiber-reinforced plastic parts which can be automated more easily without any appreciable restrictions with regard to the extent of workpiece profiling, but at the same time is nevertheless flexible in the constitution of the process and, moreover, is efficient and operationally reliable.
This object is achieved according to the invention with respect to the process by a forming press with a forming tool having a female mold and a male mold, comprising placing a blank cut off from an endless fiber mat and corresponding to a workpiece in an automated manner by an industrial robot onto the female mold, which has been moved out of the forming press and is firmly held there, in such a way that the blank can be fed after itself, by a surrounding clamping frame; spraying the blank placed in the clamping frame over an entire surface area with a specifically set amount of reactive matrix resin in a region covering the female mold by a spraying nozzle guided movably at a distance from the blank; after moving the female mold-clamping frame unit back into the forming press and closing the forming tool, while maintaining a specifically set tensile stress in the blank, draping the blank into the female mold without any folds or incipient tears, the blank is thereby formed into a desired shell form, at the same time the matrix resin is pressed into spaces between fibers and entrapped air is forced out; keeping the resin-impregnated fiber mat in a formed and pressed state for a specifically set time period and, at the same time, curing the matrix resin within the forming tool; and after opening of the forming tool, removing the plastic part and cutting off an edge of the blank lying outside the desired shell form of the workpiece, serving for stretching out the blank during the forming phase, from the workpiece and with respect to the production installation by a forming press which can be opened and closed, with a forming tool comprising a female mold and a male mold, the female mold arranged at a bottom in the forming press being surrounded on all sides by a clamping frame for the fiber mat blank placed thereon, which clamping frame is held in a spatially immovable position in relation to the female mold, a securing plane of the fiber mat blank being received in the clamping frame coinciding at least approximately with a form parting plane of the female mold; the clamping frame being designed in such a way that an edge of the blank placed on the clamping frame can be firmly clamped therewith in such a way that, when tension acts on the blank, the edge can slide out against an adjustable resistance; the female mold and the clamping frame being mounted and guided in a horizontally movable manner as a unit, which can be moved back and forth between a working position, lying in the forming press under the male mold, and a preparatory position lying completely outside the forming press; alongside the forming press, at least one industrial robot being arranged in such a way that the preparatory position of the female mold-clamping frame unit lies within a working range of the industrial robot; arranged within the working range of the industrial robot, provision containers for ready-to-hand provision of insert parts or of locally delimited preforms of fiber mats; the at least one industrial robot being provided with a hard-part gripper, which is adapted to the insert parts to be placed in, or with a textile-part gripper, which is adapted to the preforms to be placed in; arranged alongside the forming press, an unwinding station for at least one fiber mat web, which station has a holding bar, for holding a free end of the web ready, and a cutting-off device, for cutting through a drawn-off portion of the fiber mat alongside the holding bar, the holding bar likewise lying within the working range of the industrial robot; the at lease one industrial robot being provided with a gripper for drawing off a portion from the stored fiber mat web and for handling a cut-off blank; and the at least one industrial robot being provided with a controllable application nozzle for spraying a reactive matrix resin onto a partial preform placed in the female mold, which is located in the preparatory position, or onto the fiber mat blank placed there, the robot-guided application nozzle being assigned a storing and mixing station for individual components of the matrix resin.
Accordingly, a raft of different measures, which however have the common aim of achieving the underlying object, are proposed. Firstly, the production process is simplified in the sequence of steps, in that the forming of the fiber mats into the desired shell form without any folds, on the one hand, and the curing of the impregnated fiber mat in the forming tool, on the other hand, are combined into a single process step. What is more, fiber mats are only handled in the dry state, which is very conducive to a troublefree process sequence. Moreover, the process steps are constituted in such a way that all the components involved, that is any insert parts, partial preforms possibly to be placed in, the blank parts corresponding to the workpiece and the matrix resin, can be handled in an automated manner by industrial robots. The free space required for this purpose above the female mold, at which the handling operations mentioned take place, is created by the female mold being able to move horizontally and by it being temporarily moved out from the forming press.
The advantages which can be achieved with the invention are as follows: simplified and shortened process, flexible constitution of the process sequence, fold-free run of the fiber mat in spite of high profiling of the shell form of the workpiece, high degree of automation, simplified parts handling, avoidance of handling parts wetted with resin, and as a result, higher installation availability.
Of the expedient embodiments constituting the invention, at this point the one in which a portion of a fiber mat drawn off from a storage roll and corresponding to the workpiece is cut off from the wound-up fiber mat web by a high-speed rotating narrow grinding wheel, which is moved parallel to a plane of rotation and along a line between two closely neighboring holding or gripping bars and placed onto the clamping frame and the female mold (process), and the one in which the cutting-off device for the fiber mat is formed by a high-speed rotating narrow grinding wheel, which is displaceable parallel to a plane of rotation of the grinding wheel and parallel to the holding bar which is firmly holding the free end of the fiber mat web, with an adjustable advancing rate transversely over the fiber mat web (installation) are emphasized in particular, that is the cutting of the blanks corresponding to a workpiece from the stored fiber mat web by a high-speed rotating narrow grinding wheel, in particular by what is known as a cutting wheel, as are used in angle grinders for cutting metal parts or stone slabs. This cutting technique in the present application is not only virtually free from wear, but also leads to an always constantly clean cut, i.e. free from fraying and distortion, when cutting fiber mats, even after the wheel has been used for a considerable time. During the cutting of the fiber mat, the fibers are not so much ground through as rather knocked off locally, owing to inertia, on account of the high impact velocity of the abrasive particles of the narrow grinding wheel against the fixedly held fibers. If, when it is used over a considerable period of time, the abrasive particles on the circumference of the grinding wheel have become rounded and/or flattened owing to wear—this is manifested as a matter of course by the resistance during advancement of the grinding wheel increasing—the circumference of the grinding wheel can be roughened again by a dressing operation with a dressing tool, which comprises for example diamond particles, a diamond nonwoven and/or other bonded hard particles. It is conceivable at most that, after being used for a very long time, many such dressing operations may cause the grinding wheel to become worn away. It is readily possible to compensate for this by correspondingly increasing the rotational speed of the grinding wheel, so that it continues to be possible to cut the fiber mats with an approximately constant circumferential speed.
The surprisingly high service life of the cutting elements of the cutting technique proposed here for fiber mats is plausible under closer consideration. The volume wear of the geometrically undetermined “cutting edges”—in truth they are impact edges—of the proposed cutting wheels is inordinately higher than the volume wear of the conventional cutting elements of a geometrically defined form, whether they are rotating or oscillating cutters, oscillating, locally limitedly cutting shears or shearing cutter beams which cut through the entire web width with one stroke. The many abrasive particles on the overall circumference of the grinding wheel with their undetermined cutting edge geometry are available as effective cutting elements for as long as they are not flattened or rounded. In the case of conventional cutting elements, the volume wear is in any event restricted to a very narrow, direct edge region of the cutting edge of a geometrically defined form. This region is only very small, in particular when considering oscillating cutters or shears, which are slowly moved transversely over the width of the fiber mat web and through the latter. As soon as the edge of a defined form is no longer exactly sharp and free from nicks, the cutting elements begin to hook and tear, which is manifested in the form of a distorted, unclean and frayed cut edge. The cutting element then has to be exchanged for a new one.
Since, in the present case, fibers of inorganic material, such as glass fibers, ceramic fibers or carbon fibers for example, are to be used in particular, the question of wearing of the cutting device and a constantly clean cut is of particular significance. When conventional cutting devices are used, such as advancing-movable, oscillating cutters or shears or shearing cutter beams, the aforementioned fibers that are critical in terms of wear lead to relatively rapid wearing of the cutting elements and to tearing at the cut edges, which leads to an unclean deformed and/or frayed cut edge on the fiber mats. Therefore, not only material costs arise on account of the continual renewal of the cutting elements, but in this connection also indirect and direct maintenance costs on account of personnel training and operational interruptions. It is quite obvious that, even irrespective of the present production process constituted according to the invention or the production installation according to the invention, the cutting technique proposed here can be used equally advantageously anywhere where material webs made of inorganic fibers have to be cleanly cut through.
For the sake of completeness, it should be mentioned in this connection that textiles and fiber mat webs for reinforcing purposes can be cut off on the basis of fundamentally different methods with an undetermined cutting edge, for example ultrasonic cutting, high-pressure water-jet cutting, with or without particles in the water jet, or laser-beam cutting. The disadvantage of this, however, is the high technical outlay for generating the cutting energy. Moreover, cutting off material webs by means of a high-energy water jet or laser beam appears only to be acceptable in cases where it is a matter of cutting along contours of a complicated form, keeping faithfully to the contour, which is not required however in the application concerned in the present case.
Further expedient embodiments constituting the invention and their advantages can be taken from the respective subclaims; otherwise, the invention is further explained below on the basis of an exemplary embodiment represented in the drawings.